One of the degredation mechanisms observed in the operation of light emissive semiconductor devices is the development of dark line defects (absorbing region). In a double heterostructure laser these dark line defects develop in or adjacent the active layer of the laser. The defects consist of a network of dislocation loops, which are believed generally to initiate a defect or dislocation incorporated into the material during its manufacture, and that the lines developed by the condensing of point defect (intersticial, vacancies, and impurity atoms) onto existing grown-in defects. Since the degredation results from the use of the device it is further believed that the device operation somehow renders the point defect concerned very mobile.
This invention is concerned with substituting atoms of different size into the host lattice. One effect of this substitution is to lock or pin the defects or dislocations that are grown into the device by relieving some microstress present around them. It is further believed that this may reduce the mobility of the point defects during device operation. This is primarily of use in the active region of the device, but may also with advantage be applied to the layers flanking the active region.
The relief of microstress and consequent dislocation pinning that is achieved by doping may be understood by considering an edge dislocation. The stresses in the neighbourhood of the dislocation increase rapidly toward the dislocation in inverse proportion to the distance from the dislocation. The stresses have both hydrostatic and shear components. With a dopant with spherical symmetry, only the hydrostatic component is relieved. The hydrostatic stress is compressional on the side of the dislocation with the extra half plane, and tensional on the other side. If during growth of the material impurity atoms are present which would normally be incorporated into the host lattice with a concentration C.sub.o, then the concentration C around a dislocation is given by the expression C = C.sub.o exp(.beta.sin.theta./RkT) where R and .theta. are cylindrical co-ordinates, and k is Boltzmans constant, T is the absolute temperature, and .beta. is an elastic interaction coefficient proportional to the difference in radii between the impurity atom and the atom in the host lattice for which the impurity atom is substituted. Thus if the substituent is larger it is present in a greater concentration in the region of tensile stress, and present in a reduced concentration in the region of compressive stress. The gradient of impurity concentration partially relieves the stresses caused by the dislocation and also pins the dislocation in position insofar as it creates a potential energy well for the dislocation centred on the dislocation.
The concentration required to produce a significant pinning effect depends upon the degree of mismatch between the atomic sizes. In gallium arsenide, for instance, the atomic size of aluminium is so close to that of gallium that it is not suitable, although it could be used in other semiconductor materials, such as indium arsenide. On the other hand indium and antimony are sufficiently different in size from gallium and arsenic respectively for effective pinning in gallium arsenide at concentrations of not more than a few atomic percent. A substantially similar concentration in the case of phosphorus substituted for arsenic in gallium arsenide would produce a slightly smaller effect. At this concentration any shift of the wavelength of emission is normally masked by the scatter in emission wavelengths normally encountered in batches of nominally identical devices. A much larger concentration, typically about 15 atomic %, would be required to produce a significant shift in emission wavelength.